1. Field of the Invention
This invention relates to loudspeaker enclosures of the type typically classified in U.S. Patent Office Class 181.
2. Description of the Prior Art
Recent advances in semiconductor electronics have provided the consumer with a wide variety of moderately priced audio amplifiers capable of generating far more accurate audio signals than their predecessors of a decade ago. Unfortunately, this rapid growth in the electronic performance of audio amplifiers has not been paralleled by a rapid growth in the acoustical performance of audio loudspeaker enclosures. Thus the weakest link in the chain of audio reproduction is frequently the loudspeaker itself.
Ideally, a loudspeaker enclosure should possess four characteristics:
First, it should be capable of efficiently converting electrical power into acoustical power. Such conversion efficiency not only saves electrical energy, but helps minimize the amplitude distortions which can arise from the nonlinear characteristics of many audio amplifiers at their higher wattage outputs. Conversion efficiency also causes a speaker enclosure to be more responsive to the weaker audio signals generated by the amplifier, which extends the dynamic range of the enclosure. This extension of the dynamic range of the enclosure is particularly important since one of the shortcomings of reproduced music is its lack of dynamic range as compared to the range present in the original live performance. PA1 Second, a loudspeaker enclosure should be capable of generating a broad range of sound frequencies. In otherwords, it should have the longest acoustical bandwidth possible so as to be capable of accurately reproducing all sounds present in the audible sound spectrum. PA1 Third, a loudspeaker enclosure should be capable of generating all the sounds within its bandwidth with a minimum of amplitude and harmonic and intermodulation distortion. PA1 Finally, it is very desirable from the consumer's point of view that the enclosure be compact in size and relatively inexpensive.
prior art attempts to construct the ideal loudspeaker enclosure generally fall into four categories. These categories include horn loaded speaker enclosures, ported bass reflex enclosures, infinite baffle enclosures, and hybrid design enclosures which combine the principal features of two or more of the first three categories named. Unfortunately, none of the enclosures in these categories possess all four "ideal" characteristics. Rather, each of the enclosures in these categories obtains one or more of the "ideal" characteristics only by a trade off of one or more of the others.
Horn loaded speaker enclosures generally possess the first three characteristics to a high degree; i.e., they are the most efficient type of speaker known, typically having an electrical to acoustical conversion rate of between 18 and 25 percent. They are also capable of producing a broad range of frequencies with little distortion. Horn enclosures achieve these characteristics by acting as an acoustical transformer which effectively matches the high impedance at the cone of the speaker to the low impedance at the surrounding air. Structurally, this is achieved by coupling a vibrating diaphragm to the flare of a horn through a compression chamber.
However, in order to have any kind of bass bandwidth, the flare of the horn must be quite large. For example, the system disclosed in the article entitled "Horn-loading Loudspeaker Enclosure" by Plach and Williams and appearing in the May, 1952, issue of Radio and Television requires a five foot by three foot by two foot cabinet and a fifteen inch woofer to effectively produce bass frequencies of approximately 40 cycles per second. Thus efficiency, long bandwidth, and lack of distortion are achieved by trading off compact cabinet size and cost.
To solve the problem of large size, numerous structures have been devised to "fold" acoustical horns, or to utilize the room floors and walls as extensions of the horn flare in order to reduce the size of the speaker cabinet. Prior art examples of folded horns appear in U.S. Pat. Nos. 4,161,230, 3,047,090, and 2,986,220; prior art examples of horn structures using the floor and walls of a room as horn flare extensions appear in U.S. Pat. Nos. 2,994,399 and 2,825,419.
Despite these attempts, state of the art horn loaded speakers still remain relatively large, cumbersome and expensive.
By contrast, ported bass reflex enclosures need not be overly large to effectively generate sound within a broad bass bandwidth. For example, the system disclosed in U.S. Pat. No. 3,952,159 is only 18.5 inches high, 14.5 inches wide and 9.5 inches deep, and yet has an effectively flat response curve starting at about 45 cps.
Ported bass reflex enclosures are also capable of possessing the other two "ideal" characteristics of efficiency and lack of distortion to a fairly high degree. The most common type of reflex enclosure is a system which generally comprises a loudspeaker enclosure having a loudspeaker and an air duct mounted on its front face. The bass reflex typically achieves a 2 to 6 percent conversion efficiency by transmitting the back wave generated by the back of the cone surface out of the air duct in its front face in phase with the front wave generated by the front of the cone surface. Additionally, the air load placed on the back surface of the speaker cone helps to moderately reduce harmonic distortion caused by cone excursion "overhang," as well as to extend the effective bass bandwidth of the unit.
However, despite the fact that the air load on the back of the speaker cone helps reduce "overhang" harmonic distortion, the sound generated by standard bass reflex designs is often accompanied by a great deal of amplitude and other harmonic distortion due to system resonance. This high distortion characteristic caused early bass reflex systems to be nicknamed "boom boxes," characterized by "one note" bass. Thus, moderate efficiency, long bandwidth, and compact size are achieved by trading off distortionless reproduction.
Some state of the art bass reflexes attempt to reduce this distortion by placing a lot of sound absorbing baffling material along the walls of the cabinet; others attempt to reduce distortion by placing a tuned acoustical chamber into the interior of the speaker cabinet to absorb the excess sound radiation generated at the resonant frequencies of the system. Examples of such systems occur in U.S. Pat. Nos. 4,128,738 and 2,766,839, respectively. Unfortunately, efficiency suffers from the introduction of baffling material or tuned acoustical chambers into the interior of a reflex system. Hence no ideal solution to the distortion problem present in typical ported bass reflex enclosures has yet been found by the prior art.
Infinite baffle enclosures, like compression line enclosures, also possess the characteristic of being relatively compact. Additionally a well designed infinite baffle speaker will have very little harmonic distortion caused by cone "overhang" excursion, as well as a fairly long bandwidth due to the air load placed on its back surface. However, the infinite baffle speaker is grossly inefficient due to the fact that the rear wave generated by the back cone surface is entirely absorbed, and usually converts less than one-half to two percent of the electric energy it receives into sound energy. This means that more electronic amplification is necessary to produce the same amount of sound energy, which can introduce amplitude distortion due to the frequently nonlinear characteristics of audio amplifiers at high power outputs. Hence, the infinite baffle type enclosure achieves compactness, fair bandwidth capacity, and moderately low harmonic distortion by trading off efficiency, which in turn frequently reduces its capacity to produce a broad range of sound frequencies without amplitude distortion and substantially reduces the speaker's ability to achieve dynamic range.
Hybrid speaker enclosures often possess all of the four "ideal" enclosure characteristics, but only to a limited extent. For example, the speaker enclosure disclosed in U.S. Pat. No. 2,978,060 comprises a loudspeaker mounted at an angle in an open, elongated cabinet, and is described as having " . . . certain properties (a) of a horn, [and] (b) of a Hemholtz resonator or bass reflex enclosure, . . . " (col. 3, lines 15 and 16). However, this particular system also has some of the limitations associated with each, including amplitude distortion from system resonance (see Graph D, FIG. 4).
Perhaps the hybrid speaker enclosure most pertinent to the instant invention is the inventor's old Model R enclosure which is part of the prior art. The old Model R speaker enclosure comprised a rectangular cabinet 26 inches high by 13 inches wide by 121/2 inches deep which included an eight inch woofer having a back piston radiating area of 28 square inches. The woofer was acoustically mounted on the upper portion of the front enclosure wall. The old Model R enclosure also included a partitioning means diagonally extending from just under the woofer to within 11/2" from the inside surface of the back wall of the enclosure, and 11/2" from the inside surface of the bottom wall of the enclosure. This partitioning means divided the interior of the enclosure into a tuned acoustical chamber having a triangular cross section and an elongated tapered compression chamber.
The tuned acoustical chamber was defined between the lower face of the partitioning means, the side walls of the enclosure, and the inside surfaces of the front and bottom walls of the enclosure. A sound radiation entrance port was defined between the lower edge of the partitioning means and the inside surface of the bottom enclosure wall. The sound radiation entrance port had a cross sectional area of 21.56 square inches. The tuned acoustical chamber was filled with four pieces of Owens Corning RA26, 3" fiberglass baffling material.
The tapered compression chamber was defined between the upper surface of the partitioning means, the side walls of the enclosure, the top wall of the enclosure, and the back enclosure wall. The tapered compression chamber terminated in a throat defined between the lower edge of the partitioning means, and the inside surface of the back wall. Like the entrance port of the tuned acoustical chamber, the throat had a cross sectional area of 21.56 square inches. Further, the rear portion of the compression chamber directly behind the woofer was tuned with baffling material.
Finally, the old Model R had a rear transmission port defined by a gap between the rear enclosure wall and the bottom enclosure wall. This rear transmission port had a cross sectional area of 19.43 square inches. Thus the transmission port of the old Model R was 69 percent as large as the rear piston radiating area of the woofer.
In operation, the compression chamber in the old Model R speaker performed three functions. First, the air mass in compression chamber placed an air load on the back pistion radiating area of the woofer cone which helped reduce harmonic distortion due to speaker cone "overhang" excursion. Second, the chamber throat also served to load the woofer by virtue of the fact that the chamber throat was 23% smaller than the rear piston radiating area of the woofer. Finally, the tapered shape of the compression chamber served to focus or concentrate the entire vibrating column of air generated by the woofer toward the throat area at the end of the taper.
After traveling through the tapered end of the compression chamber, the pulsating air column next entered an acoustical coupling comprising a T intersection formed on top by the compression chamber throat, on one side by the back wall transmission port, on the other side by the acoustical chamber entrance port, and along its bottom side by the bottom enclosure wall. Here, the sound from the vibrating column was split between the entrance port of the acoustical chamber and the back wall transmission port.
Thus an acoustical transmission line was formed between the back piston radiating area of the speaker cone, the compression chamber, and the back enclosure wall transmission port. When the rear wall of the enclosure was placed a foot or so from one of the room walls, this transmission line was extended along the floor of the room by an acoustical structure formed by the back enclosure wall, the floor, and the room wall. The sound transmitted from the port bounced off the wall and reinforced the sound radiation generated by the front surface of the speaker cone.
The previously defined acoustical transmission line was acoustically coupled to the tuned acoustical chamber at the T intersection between the compression chamber throat, the back enclosure wall transmission port, and the acoustical chamber entrance port. The tuned acoustical chamber functioned to absorb most of the even and odd harmonics of the fundamental of the system resonance frequency.
The old Model R speaker enclosure contained only 1.75 cubic feet of volume, yet was capable of converting up to 8 percent of the electrical energy it received into acoustical energy. Additionally, the old Model R had a very broad bandwidth (extending as low as 41.2 cps) as well as a fairly flat frequency response curve. This is illustrated in FIGS. 5 and 7 a, b, c, d and e. Unfortunately some system resonance remained to audibly distort the sound generated by this enclosure.